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Oh, short term indoors would allow me to index fairly well i think. certainly better than 4 thousands of an inch, even with marine ply, true not stable longterm and likely not sub arcmin, but... would allow a first step up the ladder.
The second hirth ring could be aluminium...
As to centering error, centering is more or less automatic with hirth rings, but mounting whatever is to be indexed true to center... Another story entirely. a few measuring devices are needed, say a dial test indicator or comparator of some sort.

Next challenge is making a slide for the scribe then centering the scriber To axis, then slide to the indexer... requires quite a bit of fiddling, ok, LOT of time and effort first iterations.

Then we have mounting the index mirror on the pivot axis then the optical quality of the mirrors/filters/lenses... error sources all add up (uhm statistics say one thing, murpy´s law say other, I go for murphy)

This is a"ladder" we can climb improving the hirth ring until we reach what we need, then scribing setup then the mounting fixtures... in each iteration we improve things untill we get something usable.

I think it would likely be at least as expensive as an Astra IIIb in the end, likely few top end japan and/or german sextants if time is figured in.
If a large number of one sextant model where to be made and money, materials available... I might give it a go, but nasty things like work, friends, family takes the time and old house the money, material i might get but...